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I NASA Goddard Space Flight Center 

Stratospheric Ozone Variations Caused by Solar Proton Events 

between 1963 and 2005 

Charles H. Jackman and Eric L. Fleming 

Brief, Popular Summary of the Paper: 

Long-term variations in ozone have been caused by both natural and humankind related 
processes. In particular, the humankind or anthropogenic influence on ozone from chlorofluorocarbons 
and halons (chlorine and bromine) has led to international regulations greatly limiting the release of these 
substances. These anthropogenic effects on ozone are most important in polar regions and have been 
significant since the 1970s. Certain natural ozone influences are also important in polar regions and are 
caused by the impact of solar charged particles on the atmosphere. Such natural variations have been 
studied in order to better quantify the human influence on polar ozone. 

Large-scale explosions on the Sun near sola - maximum lead to emissions of charged particles 
(mainly protons and electrons), some of which enter the Earth’s magnetosphere and rain down on the 
polar regions. “Solar proton events” have been used to describe these phenomena since the protons 
associated with these solar events sometimes create a significant atmospheric disturbance. 

Very large solar proton events occurred in several years and caused very distinctive polar changes 
in layers of the Earth’s atmosphere known as the stratosphere (12-50 km; -7-30 miles) and mesosphere 
(50-90 km; 30-55 miles). The solar protons created hydrogen- and nitrogen- containing compounds, 
which led to the polar ozone destruction. The hydrogen-containing compounds have very short lifetimes 
and lasted for only a few days (typically the duration of the solar proton event). On the other hand, the 
nitrogen-containing compounds lasted much longer, especially in the Winter. The lifetime of these 
compounds in Winter can be quite long since winds typically transport these nitrogen-containing 
constituents from tire mesosphere and upper stratosphere to the lower stratosphere, where they can last for 
months to years depleting ozone over this time period. 

The Goddard Space Flight Center two-dimensional (latitude versus altitude) chemistry and 
transport model was used to study the long-term impact of solar proton events between 1963 and 2005 on 
stratospheric ozone. The model predicted that the very large solar proton events in 1 972, 1989, 2000, 
2001, and 2003 caused total polar ozone depletions of 1-3%, which lasted for several months to years past 
the events. These changes are modest compared to the downward trends caused by anthropogenic 
chlorine and bromine in the polar regions over the 1980-2000 time period [~3%/decade in the Northern 
Hemisphere and ~4-9%/decade in the Southern Hemisphere], however, they need to be considered in 
understanding polar changes during particular years. 



Atmospheric Chemistry and Dynamics Branch 

Code 613.3 

NASA Goddard Space Flight Center 

Greenbelt, MD 20771, U.S.A. 

( 2 -mail: Charles. H.Jackman@nasa. gov) 


Atmospheric Chemistry and Dynamics Branch 

Code 613.3 

NASA Goddard Space Flight Center 

Greenbelt, MD 20771, U.S.A. 


Abstract: Solar proton fluxes have been measured by satellites for over forty years (1963-2005). Several satellites, including the 
NASA Interplanetary Monitoring Platforms (1963-1993) and the NOAA Geostationary Operational Environmental Satellites (1994- 
2005), have been used to compile this long-term dataset. Some solar eruptions lead to solar proton events (SPEs) at the Earth, which 
typically last a few days. High energy solar protons associated with SPEs precipitate on the Earth’s atmosphere and cause increases in 
odd hydrogen (HO*) and odd nitrogen (NO y ) in the polar cap regions (>60“ geomagnetic). The enhanced HO x leads to short-lived 
ozone depletion (-days) due to the short lifetime of HO x constituents. The enhanced NO y leads to long-lived ozone changes because 
of the long lifetime of the NO y family in the stratosphere and lower mesosphere. Very large SPEs occurred in 1972, 1989, 2000, 2001, 
and 2003 and were predicted to cause maximum total ozone depletions of 1-3%, which lasted for several months to years past the 
events. These long-term ozone changes caused by SPEs are discussed below. 

1. Introduction 

Explosions on the Sun sometimes result in large fluxes of high-energy solar protons at the Earth, especially 
near the maximum period of activity of a solar cycle. This disturbed time, wherein the solar proton flux is 
generally elevated for a few days, is known as a solar proton event (SPE). Solar protons are guided by the 
Earth’s magnetic field and impact both the northern and southern polar cap regions (>60° geomagnetic 
latitude), e.g., see Jackman and McPeters (2004). These protons can impact the neutral middle atmosphere 
(stratosphere and mesosphere) and produce both HO x (H, OH, H0 2 ) and NO y (N, NO, N0 2 , N0 3 , N 2 0 5 , 
HNO 3 , H0 2 N0 2 , C10N0 2 , BrON0 2 ) constituents either directly or through a photochemical sequence (e.g., 
Swider and Keneshea, 1973; Crutzen et ah, 1975; Jackman et ah, 1980; Solomon et ah, 1981; McPeters, 
1986; Zadorozhny et ah, 1992). Ozone is also impacted by the solar protons through direct photochemical 
destruction forced by the HO x and NO y enhancements (e.g.. Weeks et ah, 1972; Heath et ah, 1977; Solomon 
etah, 1983; Jackman et ah, 1990). 

Although all sizes of SPEs can have an impact on the atmosphere, the extremely large SPEs cause the 
most pronounced changes. Several of these extremely large SPEs have occurred in the past forty years. 
Huge fluxes of high-energy protons have impacted the Earth’s atmosphere in 1972, 1989, 2000, 2001, and 
2003. The impact of SPEs over the 1963-2005 period will be discussed, concentrating particularly on the 
atmospheric effects during and after the huge SPEs. 

The paper is divided into six primary sections, including the Introduction. We discuss the very 
important solar proton measurements and their production of odd hydrogen (HO x ) and odd nitrogen (NO y ) 
in section 2. A comparison of the largest fifteen SPEs in the past four solar cycles is also undertaken in 
section 2. The GSFC two-dimensional model used to simulate the impact of the SPEs on the atmosphere is 
discussed in section 3. The short-term impact of these SPEs on ozone during and for several days after 
particular events is given in section 4. Longer term i nf luences of the SPEs on the middle atmosphere are 
discussed in section 5. Finally, the conclusions are given in section 6. 

2. Proton Fluxes; Odd Hydrogen (HO x ) and Odd Nitrogen (NO y ) Production 


Solar proton fluxes have been measured by a number of satellites in interplanetary space or in orbit around 
the Earth. The National Aeronautics and Space Administration (NASA) Interplanetary Monitoring 
Platform (IMP) series of satellites provided measurements of proton fluxes from 1963-1993. IMPs 1-7 
were used for the fluxes from 1963-1973 (Jackman et al., 1990) and IMP 8 was used for the fluxes from 
1974-1993 (Vitt and Jackman, 1996). The National Oceanic and Atmospheric Administration (NOAA) 
Geostationary Operational Environmental Satellites (GOES) were used for proton fluxes from 1994-2005 
(Jackman et ah, 2005a). 

Protons in their energy deposition process cause ionizations, dissociations, predissociations, and 
dissociative ionizations in collisions with atmospheric constituents. The protons thus produce secondaiy 
electrons, ions, excited molecules and atoms. The proton fluxes were used to compute daily average ion 
pair production profiles using an energy deposition scheme first discussed in Jackman et al. (1980). The 
scheme includes the deposition of energy by the protons and assumes 35 eV are required to produce one ion 
pair (Porter et ah, 1976). Thereby, a dataset of daily average ion pair production rates for the period 1963- 
2005 were created for use in model studies. 


Along with the ion pairs, the protons and their associated secondary electrons also produce odd hydrogen 
(HO x ). The production of HO x relies on complicated ion chemistry that takes place after the initial 
formation of ion pairs (Swider and Keneshea, 1973; Frederick, 1976; Solomon et ah 1981). Solomon et ah 
(1981) computed HO x production rates as a function of altitude and ion pair production. Each ion pair 
typically results in the production of around two HO x constituents in the upper stratosphere and lower 
mesosphere. In the middle and upper mesosphere, an ion pair is computed to produce less than two HO x 
constituents per ion pair. We include the HO x production by SPEs in our model using a look-up table (see 
Jackman et ah, 2005b) invoking the computations of Solomon et ah (1981). The HO x constituents have 
lifetimes of only hours in the middle atmosphere, therefore, any further effects on other constituents from 
the HO x group are apparent only during and shortly after an SPE. 


Odd nitrogen is produced when the energetic charged particles (protons and associated secondary electrons) 
collide with and dissociate N 2 . Following Porter et al. (1976) it is assumed that -1.25 N atoms are 
produced per ion pair. The Porter et al. (1976) study also further divided the proton impact of N atom 
production between ground state (~45% or ~0.55 per ion pair) and excited state (-55% or -0.7 per ion pair) 
nitrogen atoms. Ground state [N( 4 S)] nitrogen atoms can create other NO y constituents, such as NO, 

N( 4 S) + O 2 0NO + O (1) 

or can lead to NO y destruction through 

N( 4 S) + NO 0 N 2 + O. (2) 

Generally, excited states of atomic nitrogen, such as N( 2 D), result in the production of NO through 
N( 2 D) + 0 2 QN0 + 0 (3) 

(e.g., Rusch et al., 1981; Rees, 1989) and do not cause significant destruction of NO y . Rusch et al. (1981) 
showed that there are huge differences in the final results of model computations of NO y enhancements 
from SPEs that depend strongly on the branching ratios of the N atoms produced. We currently do not 
include any of the excited states of atomic nitrogen [e.g., N( 2 D), N( 2 P), and N + ] as computed constituents in 
our model. We use the following fairly accurate way to best represent the production of NO y constituents 
by the SPEs: Assume that 45% of the N atoms produced per ion pair result in the production of N( 4 S) 
{-0.55 per ion pair} and that 55% of the N atoms produced per ion pair result in the production of NO 
{-0.7 per ion pair}. 

The lifetime of odd nitrogen can vary dramatically depending on season and altitude. Odd nitrogen has 
a relatively short lifetime (-days) in the sunlit middle and upper mesosphere, however, lower mesospheric 
and stratospheric NO y can last for weeks past an SPE. A large portion of the SPE-produced NO y is 
conserved in a mostly dark polar middle atmosphere in the late fall and winter. This NO y can then be 
transported to lower altitudes via the general downward flowing winds during this time of year and its 
lifetime can range from months to years, if transported all the way to the middle and lower stratosphere.' 

We have quantified middle atmospheric NO y production previously (Jackman et al., 1990; Vitt and 
Jackman, 1996; Jackman et al., 2005a) for years 1963 through 2003. We add NO y computations in this 
study to these earlier calculations for years 2004 and 2005 and present the annual production from SPEs for 
the 43-year period 1963 through 2005 in Figure 1. The annual-averaged sunspot number is also shown in 
Figure 1 to illustrate the rough correlation between solar maximum periods and frequency of SPEs. 


Figure 1 . Total number of NO y molecules produced per year in the polar stratosphere and mesosphere by 
SPEs (solid histogram- left ordinate) and annually-averaged sunspot number (dashed line - right ordinate) 
for years 1963 through 2005. 

Substantial amounts of NO y were produced near solar maximum in several years. The annual global 
NO y production from solar protons is computed to be 3.7, 8.4, 6.7, 7.9, and 4.1 x 10 33 molecules for the 
very active years 1972, 1989, 2000, 2001, and 2003, respectively. These annual production rates from 
SPEs can be compared with the largest global NO y source {nitrous oxide oxidation, N 2 0+0( 1 D)} of about 
3.3 x 10 34 molecules/year (Vitt and Jackman, 1996). The SPE sources of NO y were very significant during 
these particular years for the middle atmosphere. Since the SPEs typically last only a few days, these 
impulses of NO y from SPEs can impact the polar odd nitrogen amounts substantially during brief periods. 

The fifteen largest SPEs based on NO y production in the past 40 years are given in Table 1 . 
Surprisingly, eight of them occurred in the most recent solar maximum period. 

Table 1 . Largest fifteen solar proton events in the past forty years. 

Date of SPEs 

Rank in size 

NO y production 
in the middle atmosphere 
(# of 10 33 molecules ) 

October 19-27, 1989 



August 2-10, 1972 



July 14-16, 2000 



October 28-3 1, 2003 



November 5-7, 2001 



November 9-11, 2000 



September 24-30, 2001 



August 13-26, 1989 



November 23-25, 2001 



September 2-7, 1966 



January 15-23, 2005 



Sep. 29 -Oct. 3, 1989 



Jan. 28 -Feb. 1, 1967 



March 23-29, 1991 





3. GSFC Two-dimensional Model Description and Simulations 

The latest version of the Goddard Space Flight Center (GSFC) two-dimensional (2D) atmospheric 
model was used to predict atmospheric changes caused by the solar protons. The model has been in use 
since the late 1980’s and has undergone extensive improvements over the years (Douglass et al., 1989; 
Jackman et al., 1990). The vertical range of the model, equally spaced in log pressure, is from the ground 
to approximately 90 km (0.0024 hPa) with approximately a 2 km grid spacing. Latitudes range from 85°S 
to 85°N with a 10° grid spacing. 

Fleming et al. (2002) described the methodology to compute the transport for the GSFC 2D model using 
the global winds and temperatures from meteorological data for particular years. This technique has now 
been applied using the National Centers for Environmental Prediction -National Center for Atmospheric 
Research (NCEP-NCAR) reanalysis-2 project (e.g., Kanamitsu et al., 2002). These data cover the time 
period 1958-present, and extend from the surface to 10 hPa. We have used the original NCEP analyses data 
(Gelman et al., 1986) for 10 to 1 hPa for 1979-present (climatological fields are used above 10 hPa prior to 
1979). For the mesosphere for 1-0.002 hPa, we employ the temperature measurements made by the 
Microwave Limb Sounder (MLS) onboard UARS for September 1991 through June 1997 (Wu et al., 2003). 
The 2D model residual circulation and horizontal and vertical eddy diffusion quantities are then derived 
following the methodology described in Fleming et al. (2002, 2006). 

The photochemical scheme includes all reactions that are thought to be important for ozone. The 
reaction rates, including heterogeneous rates, are taken from Sander et al. (2003). A lookup table is 
employed in computing the photolytic source term, which is then used in computation of photodissociation 
rates for atmospheric constituents (Jackman et al., 1996). The GSFC 2D chemistry solver uses the 
Atmospheric Environmental Research (AER) 2D model scheme (Weisenstein et al., 2004), which computes 
a diurnal cycle every day. The ground boundary conditions for the source gases are taken from WMO 
(2003) for the particular simulated year. The model uses chemical families and computes 55 constituents 
(Jackman et al. 2005b). 

We used the GSFC 2D model to compute two primary simulations, “base” and “perturbed,” for the 
years 1960 through 2010. The transport for years 1960-2004 is driven by NCEP products for those 
particular years, whereas the transport for the individual years 2005-2010 is an average climatology of the 
1958-2004 period. The “base” simulation includes no SPEs, whereas the “perturbed” simulation includes 
all SPEs from January 1, 1963 through December 31, 2005. The perturbation to the atmosphere was caused 
by the SPE-produced HO x and NO y enhancement. 

4. Short-term Impact on Ozone 

The ozone response due to very large SPEs is not subtle and has been observed due to numerous events to 
date (e.g., Jackman and McPeters, 2004; Lopez-Puertas et al. 2005; Seppala et al. 2006). Ozone within the 
polar caps (60-90°S or 60-90°N geomagnetic) is generally depleted to some extent in the mesosphere and 
upper stratosphere (e.g., Jackman et al. 2005b) within hours of the start of the SPE. Decreases in 
mesospheric and upper stratospheric ozone are mostly caused by SPE-induced HO x increases (see Solomon 
et al. 1981, 1983; Jackman and McPeters 1985; Jackman et al., 2005b) and last only during and for a few 

hours after the SPEs. SPE-caused NO y enhancements can also drive upper stratospheric ozone depletion, 
but do not cause significant mesospheric ozone depletion (Jackman et al., 2001). Although these short-term 
SPE impacts on ozone merit study and have helped test atmospheric models (e.g., Jackman and McPeters, 
1987), the longer-term SPE impacts on ozone are the more important component in polar stratospheric 
ozone variation and will be discussed in the next section. 

5. Long-term Impact on Ozone 

The longer-term impact of SPE-induced NO y enhancements on ozone has been known for about 30 years. 
Heath et al. (1977) showed large stratospheric ozone reductions in Nimbus-4 BUV instrument data up to 19 
days past the August 1972 events, which were probably caused by the NO y enhancements. Several other 
papers (e.g. Reagan et al., 1981; Solomon and Crutzen, 1981; Rusch et al., 1981; Jackman et al., 1990, 

1995, 2000, 2005a) studied various aspects of NO y influence on stratospheric ozone. The primary catalytic 
cycle for NO y destruction of ozone is 

NO + 0 3 0 N0 2 + 0 2 (4) 

N0 2 + 0()N0 + 0 2 (5) 

Net: 0 3 + O 0 0 2 + 0 2 (6) 

The long lifetime of the NO y constituents allows the influence on ozone to last for a number of months 
to years past the event. Figure 2 shows the model predicted temporal behavior of profile ozone (lower plot) 
and NO y (upper plot) for the polarmost Northern Hemisphere area (70°-90°N) for the time period 1963- 
2010. NO y shows enhancements between 1 and about 10% in the lower stratosphere (below 10 hPa) for 
particular years. These very large SPEs in a) August 1972; b) August-September-October 1989; and c) July 
and November 2000, September and November 2001, October 2003, and January and September 2005 
cause the NO y increases in years a) 1972-3; b) 1989-93; and c) 2000-6, respectively. 

The increased NO y led to a northern polar stratospheric ozone depletion for extended periods. SPE- 
caused depletions greater than 3% are highlighted in “dark gray” in Figure 2 (lower plot). The polar 
Southern Hemisphere shows similar behavior (not shown), however, there are differences caused by the 
seasonal differences for the occurrence of the SPEs (e.g., see discussion in Jackman et al. 2005b). SPEs 
that occur in the late fall/winter time period experience a lower amount of sunlight thus the loss process for 
NO y via 

NO + h_(< 191 nm) <) N + O (7) 

followed by 

N+NO<}N 2 + 0 (8) 

is minimal. The vertical winds are generally downward at this time of year and NO y is transported to lower 
altitudes, where photochemical loss is even less. SPEs in October 1989, November 2000, November 2001, 
October 2003, and January 2005 are thus quite important in the Northern Hemisphere. 

Enhanced levels of NO y can also lead to ozone increases (Jackman et al., 2000). This is especially hue 
in years of enhanced halogen loading. The ozone loss rate due to chlorine and bromine can be reduced 
through reactions such as 

CIO + N0 2 + M 0 C10N0 2 + M 

( 9 ) 

and BrO + N0 2 + M 0 BrON0 2 + M (1 0), 

where chlorine and bromine reservoir constituents (C10N0 2 and BrON0 2 ) are produced at the expense of 
the ozone-reducing radicals (CIO and BrO). 

Although such interference is relatively small in the NH with computed ozone increases of just over 
+0.3% at most, the average altitude of the 0.0% contour line gradually rises upwards from ~12 km in 1980 
to —15 km in 2000 as the effective equivalent stratospheric chlorine increases from 1 .8 to 3.2 ppbv over the 
same time period (see Figure 1-23 of WMO, 2003). 

NO % nhnnnf? in 7D — 90°N — from SPEs 

0 3 % change In 70-90°N - from SPEs 

■ n/ i ii I i *1 rrm in i*i i i i i-i i i i 1 1 .1 . i i. .l.arri 1 .» 1 1 \ 1 1 1 1 1111 w 

I960 1970 1980 1990 2000 2010 


Figure 2. Model computed percentage changes in NO y and 0 3 for the polar Northern Hemisphere area (70- 
90°N) for 1963-2010 resulting from SPEs in 1963-2005. Contour levels for NO y (top plot) are +1%, +3%, 
and +10%. The “light gray” and “dark gray” highlighted areas for NO y indicate increases from 3% to 10% 
and >10%, respectively. Contour levels for 0 3 (bottom plot) are -3%, -1%, -0.3%, 0%, and +0.3%. The 
“light gray” and “dark gray” highlighted areas for 0 3 indicate decreases from 1% to 3% and >3%, 
respectively. These changes were computed by comparing the “perturbed” to the “base” simulation. 

The impact on total ozone is shown in Figure 3. Both hemispheres had extended periods of depleted 
ozone from 1-3% in 1972, 1989-90, and 2000-3. These changes are modest compared to the downward 
trends caused by halogen loading in the polar regions over the 1980-2000 time period [~3%/decade in the 
NH and -4-9%/decade in the SH, WMO (2003)], however, they need to be considered in understanding 
polar changes during particular years. 

Total Ozone % change - from SPEs 

I960 1970 1980 1990 2000 2010 


Figure 3. Model computed percentage total ozone changes for 1963-2010 resulting from SPEs in 1963- 
2005. Contour intervals are -1%, - 0 . 3 %, and -0.1%. The “light gray” and “dark gray” highlighted areas for 
indicate decreases from 0.3% to 1% and >1%, respectively. These changes were computed by comparing 
the “perturbed” to the “base” simulation. 

6. Conclusions 

Several very large SPEs have occurred over the 43-year time period 1963-2005. These events, which 
occurred in 1972, 1989, 2000, 2001, and 2003, have led to significant polar enhancements in HO x and NO y . 
The HO x enhancements led to short-term mesospheric and upper stratospheric ozone decreases, whereas the 
NO y enhancements led to polar total ozone depletions of 1-3% lasting several months to years past the 
events. The NO y enhancements were also found to lead to small ozone enhancements in the lowermost 
stratosphere because of interference with the ozone loss cycles for the chlorine and bromine constituents. 

Acknowledgments: This work was supported by the NASA Living With a Star Targeted Research and Technology Program and the 
NASA Atmospheric Composition Data and Analysis Program. We thank the Interplanetary Monitoring Platform and NOAA 
Geostationary Operational Environmental Satellite teams for providing the solar proton flux data. 

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